Tungsten inert gas welding of aluminum, aluminum alloys and similar metal is well known and a number of square wave welders have been developed for A.C. welding of such materials. When A.C. welding of aluminum, there is some difficulty caused by the fact that the electrode and metal workpiece have different abilities to emit electrons. The electrode emits electrons more readily than the metal workpiece, especially for aluminum, aluminum alloys and similar metals. Thus, during the A.C. current half cycle wherein the welding arc is maintained by current flow from the electrode (electrode negative) to the workpiece (referred to in the art as the straight polarity or weld half cycle) there is little resistance caused by the characteristics of the electrode or the workpiece to the formation or maintenance of the arc. In contrast, during the A.C. current half cycle wherein the welding arc is maintained by electron flow from the workpiece to the electrode (referred to in the art as the reverse polarity or clean half cycle) the tendency of the workpiece, particularly aluminum or aluminum alloys, to emit electrons poorly frequently results in non-conduction or poor conduction of current during this half cycle. This phenomenon is referred to as arc rectification and results in somewhat reduced quality welds. Accordingly, the welding field has sought welding current supplies which will reliably and inexpensively solve the problem of arc rectification and provide a stabilized welding arc for TIG A.C. arc welding of aluminum, aluminum alloys and similar metals.
The use of current waveforms having square shapes has reduced the difficulties experienced in TIG welding of aluminum by increasing the rate of di/dt at zero crossings; however, establishing the arc during the transition between electrode negative and electrode positive has still been somewhat unstable. Consequently, it is not uncommon to provide square wave welders with high frequency pulses of current having a relatively high voltage at the time of arc current reversal caused by switching the square wave power supply from one current polarity to the other current polarity. The high frequency, high voltage pulses are induced into the welding lead by a high frequency transformer where the secondary of the transformer is part of the welding circuit. When the arc current is to be reversed in polarity, a high frequency pulse, usually controlled by a microprocessor or other control device, causes one or more high frequency pulses across the arc gap to assure the creation of an arc in the reverse current direction. This prior art solution for establishing the arc as the current is reversed sometimes fails to cause an arc upon switching from straight to reverse polarity and, thus, results in an unstable arc. Such use of a high frequency starting voltage to force conduction of electrons from the aluminum workpiece to the tungsten electrode upon switching of the welding current from straight to reverse polarity does improve the performance over prior art having no such high frequency control arrangement; however, arc rectification can still occur at the current reversals of the square wave A.C. TIG welder.
The problem of arc stability is quite prevalent in an A.C. TIG welder of the type shown in the Risberg and Winn patents. The present invention has been developed for the purpose of controlling arc stabilization by assuring the creation of an electric arc when switching from straight polarity to reverse polarity in an A.C. TIG welder of the type shown in Risberg and Winn. These two patents, which disclose power supplies to which the present invention is particularly directed, include SCR switching means for causing the reversal of welding current or arc current. This type of A.C. TIG welder power supply has a tendency to free wheel at current reversals because both SCR circuits are conducting at the same time and the energy stored within the squaring inductor freewheels through all the SCRs of the squaring circuit. Such freewheeling stops when the discontinued current path is rendered non-conductive by commutation of its SCR or pair of SCRs. In this type of power supply, not only is the arc unstable, even with high frequency starting, but also freewheeling at current reversal prevents a rapid conversion from one polarity to the other polarity.
These problems are solved by the present invention which is directed toward a control circuit of an A.C. TIG welder, especially of the type having a four SCR bridge with an intermediate D.C. choke for current squaring.